With the widespread use of multimedia in recent years, a write-once optical disk, i.e., CD-R, and a rewritable optical disk, i.e., CD-RW, have come into widespread use quickly, and optical disks having larger capacities, such as a write-once optical disk, i.e., DVD-R, and rewritable optical disks, i.e., DVD-RW and DVD-RAM, are put to practical use and beginning to be used widely. In order to record information, each of these optical disks is used with an information recording/reproducing apparatus including an optical pickup system having a semiconductor laser as a light source. Various technological developments have been carried out to enhance and stabilize the recording and reproduction performance of these information recording/reproducing apparatuses. A semiconductor laser power control technology is one of the technologies, and numerous systems have been proposed. In particular, in the case of the write-once optical disk, since no data can be rewritten, it becomes important how a stable recording state can be maintained. The laser power control systems includes, for example, a system referred to as APC (Auto Power Control) wherein part of the amount of outgoing laser light is detected and laser power is controlled so that outgoing power is maintained constant, and a system referred to as R-OPC (Running-Optimum Power Control) wherein the amount of reflection light from a recording medium is detected during recording, and laser power is controlled so that the amount of reflection light from the recording mark areas of the recording medium becomes a predetermined amount of light.
For example, as disclosed in Japanese Patent Application No. 2000-295734, a system is proposed wherein laser power is controlled in combination with APC and R-OPC by using the recording waveform of a multi-pulse train with pulses having different widths for mark areas.
FIG. 7 is a block diagram showing a prior art information recording apparatus of carrying out laser power control in combination with APC and R-OPC, and FIG. 8 shows the detection signal waveforms for APC and the detection signal waveforms for R-OPC in the prior art information recording apparatus that uses the recording waveform of a multi-pulse train with pulses having different widths for mark areas.
Numeral 101 designates an optical disk capable of recording and reproducing information. Numeral 102 designates a spindle motor of rotating the optical disk 101.
Numeral 103 designates a laser diode capable of emitting light by using a multi-pulse train with pulses having different widths for mark areas during recording on the optical disk 101. Numeral 104 designates a beam splitter of separating the outgoing light of the laser diode 103 and the return light from the optical disk 101. Numeral 105 designates an objective lens of gathering laser light in recordable or reproducible areas of the optical disk 101.
Numeral 106 designates a light amount monitoring photodetector of detecting part of the outgoing light emitted from the laser diode 103. Numeral 107 designates an I/V conversion circuit of converting the current output of the light amount monitoring photodetector 106 into a voltage.
Numeral 108 designates a low-pass filter (LPF) of attenuating the frequency band of the output of the I/V conversion circuit 107. Numeral 109 designates a voltage amplifier (AMP) of amplifying the current output of the light amount monitoring photodetector 106. Numeral 110 designates a sample-and-hold circuit (S/H) of sampling and holding the output of the LPF 108 at predetermined timing. Numeral 111 designates a sample-and-hold circuit (S/H) of sampling and holding the output of the AMP 109 at predetermined timing.
Numeral 112 designates an mp detector of detecting the output of the S/H 110 as the average power of recording light emission for a multi-pulse train.
Numeral 113 designates an sp detector of detecting the output of the S/H 111 as the bias power of recording light emission for a multi-pulse train.
Numeral 114 designates a peak power control circuit of controlling the peak power of the recording light emission for a multi-pulse train.
Numeral 115 designates a bias power control circuit of controlling the bias power of the recording light emission for a multi-pulse train. Numeral 116 designates an LD drive circuit of driving the laser diode 103 to emit light by using power and a multi-pulse train with pulses having different widths controlled by the peak power control circuit 114 and the bias power control circuit 115.
Numeral 117 designates a plurality of return light detecting photodetectors of detecting return light from the optical disk 101. Numeral 118 designates a plurality of I/V conversion circuits of converting each of the current outputs of the plurality of return light detecting photodetectors 117 into a voltage.
Numeral 119 designates an RF adder of adding the outputs of the plurality of I/V conversion circuits 118. Numeral 120 designates a low-pass filter (LPF) of attenuating the frequency band of the output of the RF adder 119. Numeral 121 designates a voltage amplifier (AMP) of amplifying the output of the LPF 120. Numeral 122 designates a sample-and-hold circuit (S/H) of sampling and holding the output of the LPF 120 at predetermined timing. Numeral 123 designates a sample-and-hold circuit (S/H) of sampling and holding the output of the AMP 121 at predetermined timing.
Numeral 124 designates an MP detector of detecting the output of the S/H 122 as the average amount of return light from the mark areas of the optical disk 101 during recording.
Numeral 125 designates an SP detector of detecting the output of the S/H 123 as the amount of return light from the space (non-mark) areas of the optical disk 101 during recording.
Numeral 126 designates a B/A calculation circuit of calculating a B/A value used as a parameter required for the R-OPC operation from the output of the MP detector 124 and the output of the SP detector 125. Numeral 127 designates a CPU of calculating the correction amount of the peak power on the basis of the output of the B/A calculation circuit 126 and of issuing a target peak power command to the peak power control circuit 114.
In accordance with the configuration described above, the operation of carrying out laser power control in combination with ARC and R-OPC will be described below by using the detection signal waveforms for APC and the detection signal waveforms for R-OPC shown in FIG. 8.
In the case when the record data shown in FIG. 8(a) is converted into a multi-pulse train with pulses having different widths shown in FIG. 8(b) and when the laser diode 103 is controlled by predetermined peak power and predetermined bias power, first, as the APC operation, the light emission waveform shown in FIG. 8(c) is detected by the light amount monitoring photodetector 106, the output of the trailing end of the multi-pulse train becomes the mp level shown in FIG. 8(d) by the LPF 108, and mp is sampled and held by the S/H 110 and detected by the mp detector 112 as average power formed of the pulse width ratio of the multi-pulse train and converted from the pulse width ratio of the multi-pulse train, whereby the peak power is detected.
Furthermore, the sp level shown in FIGS. 8(c) and 8(d) is amplified by the AMP 109 and then sampled and held by the S/H 111 and detected by the sp detector 113 as bias power. The peak power control circuit 114 and the bias power control circuit 115 control the laser power so that the peak power and the bias power obtained from mp and sp become predetermined values.
Next, the R-OPC operation will be described. Although the control is carried out on the basis of the predetermined power values by the APC operation, an optimum recording power value differs depending on recording areas and recording states owing to variations in recording sensitivity because of differences in recording areas on the recording face of the optical disk 101. Hence, in addition to the APC operation, another power control operation is required to be carried out so that the optimum recording power is obtained in the states wherein recording is performed.
This power control operation is R-OPC, and the correction amount of the power is calculated while the return light during recording is detected. FIG. 8(e) shows the waveform of return light during recording, generated from the return light detecting photodetectors 117, the I/V conversion circuits 118 and the RF adder 119.
“A” in FIG. 8(e) designates the maximum level of the return light in a state wherein no mark is formed in the mark areas of the recording face of the optical disk 101 during light emission at the peak power, and “B” designates the maximum level of the return light in a mark forming state during light emission at the peak power. “A” is substantially proportional to the reflectivity of the non-recording areas of the optical disk 101 and the peak power, and “B” is based on the relationship between the reflectivity being different depending on the forming state of the recording marks of the optical disk 101 and the peak power. The value (B/A) obtained by dividing B by A is detected, and the peak power is controlled so that a predetermined B/A value is obtained.
However, as the pulse width of the multi-pulse train becomes shorter, it is difficult to directly detect the A and B values; just like the APC operation, as shown in FIG. 8(f), the output of the trailing end of the multi-pulse train becomes an MP level by the LPF 120, and MP is sampled and held by the S/H 122 and detected by the MP detector 124 as an average return light amount formed of the pulse width ratio of the multi-pulse train and converted from the pulse width ratio of the multi-pulse train, whereby the B value shown in FIG. 8(e) is detected.
Furthermore, the SP level shown in FIGS. 8(e) and 8(f) is amplified by the AMP 121 and then sampled and held by the S/H 123 and detected by the SP detector 125 as the amount of return light at the bias power, and the A value is detected from the ratio between the peak power and the bias power. After the B/A value is calculated, it is compared with the predetermined B/A value at the optimum recording power supplied to the optical disk 101, a peak power correction amount of obtaining the above-mentioned predetermined B/A value is obtained by the CPU 127, and the correction of the peak power is commanded to the peak power control circuit 114, whereby proper power control is carried out. FIG. 8(g) shows examples of recording marks recorded by the laser power controlled by the above-mentioned APC and R-OPC operations.
As a result, even when the recording sensitivity varies depending on the difference in the recording areas of the recording face of the optical disk 101, the mark forming state during recording can always be judged by using the return light and the B/A value, whereby it is possible to control the laser power so that an optimum recording state can be attained.
However, when stresses, such as defocusing, off-tracking and tilting, are changed during recording, not only the optimum recording power is changed, but also the detection of the return light amount is affected, and the B/A value used as the control target value of R-OPC is changed with respect to the optimum recording power supplied to the optical disk.
FIG. 9 shows an example of the relationship of the change of the optimum B/A value depending on the change of stress. This relationship is discovered by the inventors. FIG. 9 shows the dependence of the B/A value for the recording power on radial tilting indicating tilting in the radial direction of the disk, and shows that the B/A value for the optimum recording power increases as radial tilting increases.
The recording peak power control operation by R-OPC will be described by using FIG. 14 showing the dependence of the R-OPC detection signal on power. {circle around (1)} in FIG. 14 designates a point that indicates the optimum recording power Po and the R-OPC detection signal B/Ai at Po in a characteristic curve A indicating the relationship between the power and B/A immediately before or after recording, and B/Ai is a control target value of R-OPC during recording. {circle around (2)} designates a point that indicates the optimum recording power Po at the time of the characteristic curve A and the R-OPC detection signal B/An1 on a characteristic curve B in the case when the change of the stress is not caused but only the variation of the disk sensitivity is caused. {circle around (2)}′ designates a point that indicates the true optimum recording power P1 and the R-OPC detection signal B/Ai at P1 in the characteristic curve B, and the B/Ai is the same B/A value as that of {circle around (1)} of the characteristic curve A. {circle around (3)} designates a point that indicates the optimum recording power Po at the time of the characteristic curve A and the R-OPC detection signal B/An2 in a characteristic curve C in the case when tilting, one of the stresses, is caused significantly. {circle around (3)}′ designates a point that indicates the true optimum recording power P2′ and the R-OPC detection signal B/Ai′ at P2′ in the characteristic curve C. {circle around (3)}″ designates a point that indicates recording peak power P2 for B/Ai at {circle around (1)} and {circle around (2)}′ in the characteristic curve C. In the R-OPC operation in the characteristic curve B wherein the change of stress is not caused, since the R-OPC detection signal B/An1 at the power Po during recording is deviated from the control target value B/Ai, recording is continued while the power is changed to the power P1 obtained by adding a correction amount Pc1 to Po so that convergence is performed to the control target value. Since P1 is the optimum recording power value, the quality of recording is maintained.
Next, in the R-OPC operation in the characteristic curve C wherein tilting, one of the stresses, is caused significantly, since the R-OPC detection signal B/An2 at the power Po during recording is deviated from the control target value B/Ai, recording is continued while the power is changed to the power P2 obtained by adding a correction amount Pc2 to Po so that convergence is performed to the control target value. Since P2 is larger than the optimum recording power value P2′, the quality of recording is not maintained.
In other words, when power control is carried out with the B/A value used as the R-OPC control target value fixed at a constant value, control is carried out by using power excessively larger than the optimum recording power.
As a result, stable recording becomes difficult, and data reproduction after recording is affected significantly.